1. Field of the Invention
The present invention relates to an optical recording medium for optically recording, reproducing or erasing information therein. Specifically, the present invention relates to an optical recording medium which is capable of performing a high density recording operation and a tracking servo circuit adapted for tracking operation of the optical recording medium.
2. Description of Related Art
Conventionally, there are proposed not only an optical recording medium of a type in which a tracking servo guide groove is formed on a substrate thereof for realizing a push-pull type tracking operation, but also an optical recording medium of a type in which wobbling pits are formed on a substrate thereof for realizing a sample-servo type tracking operation.
An essential part of one example of a conventional optical recording medium of the sample-servo tracking type is shown in FIG. 1. The conventional sample-servo tracking type optical recording medium 1000 includes a disk-shaped substrate in which tracks 1002 are formed in concentrical or spiral fashion around a center point (not shown in FIG. 1) of the disk-shaped substrate. It is noted that the center point of the disk-shaped substrate is positioned downwardly of the tracks 1002 in FIG. 1. In other words, upward direction in FIG. 1 indicate radially outwardly extending direction of the disk-shaped substrate.
The optical recording medium 1000 has a plurality of sector areas 1010, each sector area 1010 consisting of a servo area 1006 adapted for a tracking servo operation and a data area 1009 adapted for a data recording operation. In the servo area 1006, there are formed a pair of wobbling pits 1004 and 1004' which are arranged in a wobbling fashion along each track 1002. More specifically to say, a wobbling pit 1004a is formed radially outwardly of one track 1002a with a distance between the wobbling pit and the track having a predetermined value. The wobbling pit 1004'a is formed radially inwardly of the track 1002a. A distance between the wobbling pit 1004'a and the track 1002a is equal to the distance between the wobbling pit 1004a and the track 1002a. The wobbling pits 1004a and 1004'a are apart from each other in a track extending direction. Each of the wobbling pits 1004a and 1004'a includes a concave or convex portion formed on a substrate of the optical recording medium on which a recording layer and a protective layer are laminated. In the data area 1009, a recording bit is formed, so that an information is recorded. It should be further noted that an address pit for supplying an address information and a clock pit for supplying a clock information are also formed in the servo area 1006, though those pits are not expressly shown in FIG. 1.
The sample-servo type tracking operation achieved by the above-described optical recording medium 1000 will be described below.
The optical recording medium 1000 is rotated so that laser beam spot 1001 irradiated on the optical recording medium 1000 is moved relatively with respect to the recording medium 1000 along the track extending direction in a rightward direction in FIG. 1. The laser beam is reflected by the optical recording medium, and an intensity of the reflected laser beam is detected by a photo-detecting unit of a sample-servo tracking circuit (not shown in the drawing). In the case where the laser beam spot 1001 is irradiated on the recording medium 1000 at a portion where no wobbling pit is formed, the intensity of the laser beam reflected from the recording medium is at a predetermined level (which will be referred to as a "background level", hereinafter). When the photo-detecting unit detects that the intensity of the reflected light is the background level, it therefore outputs a signal reprsenting the background level. In the case where the laser beam spot is irradiated onto the wobbling pit 1004 or 1004', however, since the laser beam is diffracted by the wobbling pit, the intensity of the reflected laser beam is decreased from the background level. When the photo-detecting unit detects that the intensity of the reflected light is decreased from the background level, the photo-detecting unit generates a signal representing the amount with which the intensity of the reflected laser beam is decreased from the background level. (The signal generated by the photo-detecting unit will be referred to as a "intensity decreased amount representing signal", hereinafter.) Accordingly, in the case where the laser beam spot 1001 is moved along the track 1002a for the tracking operation with respect to the track 1002a, the photo-detecting unit generates the intensity decreased amount representing signals, at the time when the laser beam spot 1001 reaches the wobbling pits 1004a and 1004'a. In the case where the laser beam spot 1001 is moved exactly on the track 1002a, the intensity decreased amount signals 1020 and 1021 generated at the timings when the laser beam spot 1001 reaches the wobbling pits 1004a and 1004'a have values which are equal to each other, as shown in FIG. 2(a). In the case where the laser beam spot 1001 is moved along a line which is positioned radially outwardly of the track 1002a, i.e., along a line positioned above the track 1002a in FIG. 1, the value of the signal 1020 generated at the timing when the laser beam spot 1001 reaches the wobbling bit 1004a becomes larger than that of the signal 1021 generated at the timing when the laser beam spot reaches the wobbling bit 1004'a, as shown in FIG. 2(b). On the other hand, in the case where the laser beam spot 1001 is moved along a line positioned radially inwardly of the track 1002a, i.e., along a line positioned below the track 1002a in FIG. 1, the value of the signal 1020 becomes lower than that of the signal 1021, as shown in FIG. 2(c).
In the sample-servo tracking circuit (not shown in the drawing), the signals 1020 and 1021 generated from the photo-detecting unit are sampled and held in a sample-and-hold circuit and are supplied to a differential amplifier. The differential amplifier outputs a differential signal representing a difference between the values of the signals 1020 and 1021. The differential signal outputted from the differential amplifier serves as a tracking error signal representing a shift amount of the laser beam spot from the track. A tracking servo motor is controlled in accordance with the differential signal outputted from the differential amplifier, so that a value of the differential signal may become zero, to thereby allow the laser beam spot 1001 to be moved exactly on the track 1002a.
When the laser beam spot 1001 is irradiated onto the servo area 1006 of the optical recording medium, the above-described sample-servo tracking operation is achieved so that the tracking servo motor may be controlled into such a condition as for allowing the laser beam spot 1001 to be moved exactly along the track 1002. When the laser beam spot 1001 is irradiated onto the data area 1009, data processing operation (i.e., data recording operation, data reproducing operation, and data erasing operation, etc.) is achieved, while the condition of the servo tracking motor obtained through the tracking operation in the servo area 1006 is maintained to be fixed to that which has been controlled through the tracking operation in the servo area 1006.
The sample-servo tracking operation and the data processing operation are conducted for each sector area 1010 of the optical recording medium, so that the data processing operation can be properly achieved in the data area 1009 with the laser beam being maintained to be moved exactly along the track.
In the case where a track pitch, i.e., a distance between each two adjacent tracks is lowered to enhance the recording density, however, a wobbling pit 1004b provided for a track 1002b which is positioned next to the track 1002a inadvertantly affects the wobbling pit 1004a. More specifically to say, as shown in FIG. 1, the wobbling pit 1004'a is formed at a position which is placed radially inwardlly of the track 1002a and is apart from the track with a predetermined distance. The wobbling pit 1004'a serves to cooperate witch the wobbling pit 1004a to achieve a tracking operation with respect to the track 1002a, as described above. The wobbling pit 1004b is formed at a position which is placed radially outwardly of the track 1002b which is placed in a radially inner side with respect to the track 1002a and is next thereto. The distance between the wobbling pit 1004b and the track 1002b is equal to the distance between the wobbling pit 1004'a and the track 1002a. The wobbling pit 1004b serves to cooperate with a wobbling pit 1004'b to achieve a tracking operation with respect to the track 1002b. The wobbling pits 1004'a and 1004b are apart from each other along a radial direction of the disk-shaped optical recording medium with a distance which is smaller than the distance between the tracks 1002a and 1002 b. In the case where the track pitch is small, therefore, the laser beam spot 1001 irradiated on the track 1002a for performing a tracking operation with respect to the track 1002a is liable to be erronesouly partly irradiated onto the wobbling pit 1004b. Such a partly irradiation of the laser beam onto the wobbling pit 1004b affects the intensity of the laser beam reflected from the optical recording medium, as a result of which the value of the signal 1020 is erroneously shifted from its correct value which will be obtained in the case where the laser beam spot is irradiated only onto the wobbling pit 1004a. The tracking error signal obtained based on the signal 1020 thus affected by the wobbling pit 1004b fails to correctly represent the shifted amount of the laser beam spot 1001 from the track 1002a. Accordingly, the tracking operation is improperly achieved, and the data processing operation is erroneously achieved with the laser beam spot 1001 being shifted from the track.